There are certain TLD's at enom (and probably other registrars too) that can only be set to auto-renew at the registrar, and cannot be renewed manually via WHMCS/API.
Here is more on Auto-renew Only domainshttps://help.enom.com/hc/en-us/articles/115003483288-Auto-renew-only-domains
in these cases, because the expiry date gets synced automatically after it is renewed by enom, WHMCS then won't invoice the customer for the domain, because it is no longer due for renewal.
This could potentially be causing providers to lose money on domain renewals not being billed.
the only solution currently is to disable "sync next due date" across the entire system, which is obviously not viable.
WHMCS should be able to handle these TLD's.
The simple solution would be to turn off "sync next due date" automatically for these TLD's so the client still gets billed.